As a method of determining a residual capacity of a battery, there are known a method involving detecting a charge/discharge current (hereinafter simply referred to as “current”) and calculating the residual capacity based on a current integrated value obtained by integrating the detected currents, and a method of calculating the residual capacity based on an open circuit voltage. The method based on the current integrated value is capable of obtaining a stable residual capacity even when a load drastically fluctuates, for example, but a current detection error is liable to accumulate. On the other hand, the method based on the open circuit voltage can determine the residual capacity accurately, but the calculation value significantly fluctuates when the load drastically fluctuates, for example.
In view of this, in the residual capacity calculating apparatus of JP 2006-38495 A, in order to improve the accuracy of estimating the residual capacity, a residual capacity determined based on a current integrated value and a residual capacity determined based on an open circuit voltage that is estimated based on an internal resistance are subjected to weighted combination with use of a weight that is set based on a current change rate.
With this, by taking advantages of the calculation method based on the current integrated value and the calculation method based on the open circuit voltage, the accuracy of estimating the residual capacity is improved.